text: //===-- runtime/unit-map.h --------------------------------------*- C++ -*-===//
  //
  // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  // See https://llvm.org/LICENSE.txt for license information.
  // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  //
  //===----------------------------------------------------------------------===//
  
  // Maps Fortran unit numbers to their ExternalFileUnit instances.
  // A simple hash table with forward-linked chains per bucket.
  
  #ifndef FORTRAN_RUNTIME_UNIT_MAP_H_
  #define FORTRAN_RUNTIME_UNIT_MAP_H_
  
  #include "lock.h"
  #include "unit.h"
  #include "flang/Common/constexpr-bitset.h"
  #include "flang/Runtime/memory.h"
  #include <cstdint>
  #include <cstdlib>
  
  namespace Fortran::runtime::io {
  
  class UnitMap {
  public:
    ExternalFileUnit *LookUp(int n) {
      CriticalSection critical{lock_};
      return Find(n);
    }
  
    ExternalFileUnit &LookUpOrCreate(
        int n, const Terminator &terminator, bool &wasExtant) {
      CriticalSection critical{lock_};
      auto *p{Find(n)};
      wasExtant = p != nullptr;
      return p ? *p : Create(n, terminator);
    }
  
    // Unit look-up by name is needed for INQUIRE(FILE="...")
    ExternalFileUnit *LookUp(const char *path) {
      CriticalSection critical{lock_};
      return Find(path);
    }
  
    ExternalFileUnit &NewUnit(const Terminator &);
  
    // To prevent races, the unit is removed from the map if it exists,
    // and put on the closing_ list until DestroyClosed() is called.
    ExternalFileUnit *LookUpForClose(int);
  
    void DestroyClosed(ExternalFileUnit &);
    void CloseAll(IoErrorHandler &);
    void FlushAll(IoErrorHandler &);
  
  private:
    struct Chain {
      explicit Chain(int n) : unit{n} {}
      ExternalFileUnit unit;
      OwningPtr<Chain> next{nullptr};
    };
  
    static constexpr int buckets_{1031}; // must be prime
    int Hash(int n) { return std::abs(n) % buckets_; }
  
    ExternalFileUnit *Find(int n) {
      Chain *previous{nullptr};
      int hash{Hash(n)};
      for (Chain *p{bucket_[hash].get()}; p; previous = p, p = p->next.get()) {
        if (p->unit.unitNumber() == n) {
          if (previous) {
            // Move found unit to front of chain for quicker lookup next time
            previous->next.swap(p->next); // now p->next.get() == p
            bucket_[hash].swap(p->next); // now bucket_[hash].get() == p
          }
          return &p->unit;
        }
      }
      return nullptr;
    }
    ExternalFileUnit *Find(const char *path);
  
    ExternalFileUnit &Create(int, const Terminator &);
  
    Lock lock_;
    OwningPtr<Chain> bucket_[buckets_]{}; // all owned by *this
    common::BitSet<64> busyNewUnits_;
    OwningPtr<Chain> closing_{nullptr}; // units during CLOSE statement
  };
  } // namespace Fortran::runtime::io
  #endif // FORTRAN_RUNTIME_UNIT_MAP_H_
  